Update Doc/tutorial/classes.rst

Author: terryjreedy

Doc/tutorial/classes.rst

@@ -359,7 +359,7 @@ Usually, a method is called right after it is bound::
 
    x.f()
 
-If `x = MyClass()`, as above, this will return the string ``'hello world'``.
+If :source:`x = MyClass()`, as above, this will return the string ``'hello world'``.
 However, it is not necessary to call a method right away: ``x.f`` is a method
 object, and can be stored away and called at a later time.  For example::